He drew a lot during his childhood and adolescence, but left school with few qualifications to his name.

Later Kidby was trained as a dental technician making false teeth, but abandoned this to become a freelance artist in 1986. He produced artwork for a variety of products ranging from video film sleeves to computer game packaging.

In 1995 he started working exclusively for Terry Pratchett’s Discworld series, and his Discworld art, and many descriptions of the pieces, are included in The Pratchett Portfolio (1996) and The Art of Discworld (2004). These showcase full versions of work and some paintings previously unseen. He also illustrated The Last Hero, a Discworld "fable" and number one bestseller in the UK, selling more than 235,000 copies.